Methanol is produced from synthesis gas (syngas), a mixture of hydrogen (H.sub.2). carbon monoxide (CO), and carbon dioxide (CO.sub.2). The stoichiometry of the methanol synthesis reactions indicates that the desired molar reactor feed composition is given by the equation: EQU R=(H.sub.2 -CO.sub.2)/(CO+CO.sub.2)=2.0
However, reaction kinetics and system control dictate that the optimum ratio is actually R=2.1 or higher. Gas with R=2.0 to 2.1 is called "balanced" gas, i.e. balanced stoichiometrically, and has a typical composition of 19% CO, 5% CO.sub.2, 55% H.sub.2, and 21% CH.sub.4 --N.sub.2.
Syngas is commonly made by the reforming of methane or other hydrocarbons, which gives a hydrogen-rich gas well-suited for methanol synthesis (e.g., a typical methanol syngas produced by steam reforming of methane has a composition of 15% CO, 8% CO.sub.2, 73% H.sub.2, 4% CH.sub.4 --N.sub.2, R=2.8). Currently 70 to 75% of the world's methanol comes from reformed natural gas, however, because of the instability of the oil market, liquid hydrocarbons and natural gas are not always readily available or available at an inexpensive cost. An alternative and abundant resource is coal, which can be converted to syngas in a coal gasifier such as the advanced, high-temperature coal gasifiers developed by Texaco, Dow, Shell, and British Gas/Lurgi.
Coal-derived syngas can be used as gas turbine fuel in an integrated gasification combined cycle (IGCC) electric power plant. Because of the daily cyclical demand for power, a primary concern in such a facility is load-following flexibility. To accomplish this flexibility, either the front end of the IGCC plant must be built for peak capacity, or extra fuel must be imported during peak periods (called peak shaving). The former is an expensive and inefficient option. The latter, although somewhat less expensive, can be improved by producing and storing the fuel on-site. One solution to this problem is the on-site production of methanol as the peak-shaving fuel.
In an IGCC facility without methanol coproduction, the syngas is combusted in a gas turbine to produce electricity. The turbine exhaust/stack gas is used to generate and superheat steam in an integrated heat recovery system, and this steam is also used to generate electricity. In a coproduction facility, the syngas is first passed through a methanol synthesis reactor to convert a portion to methanol; the remaining syngas is fed to the gas turbine for power production. The methanol is stored as peak-shaving fuel, which is used to augment the feed to the gas turbine during periods of high power demand. This scheme is attractive because the load on a power plant varies over a wide range, and it is more economical to feed the stored methanol than to build peak-shaving capacity into the front end of the facility.
Unfortunately, coal-derived syngas from advanced gasifiers used in IGCC plants is CO-rich (e.g., a Texaco gasifier syngas has a typical composition of 35% H.sub.2, 51% CO, 13% CO.sub.2, 1% CH.sub.4 --N.sub.2 ; R=0.34), unlike the hydrogen-rich syngas from reformed hydrocarbons. The problem is that converting this gas to methanol by conventional methods is expensive and complicated because several pretreatment steps are required to balance the gas prior to methanol synthesis.
Conceptual IGCC coproduct plants have been designed with gas-phase and with liquid-phase methanol synthesis reactors. With a gas-phase reactor, the main syngas stream from the gasifier is divided into two parts: approximately 75% goes directly to the gas turbine, and the remaining 25% goes to the methanol synthesis section. This latter stream is further divided, approximately 67% being mixed with steam and sent to a high temperature shift reactor (HTS). After shift, the CO.sub.2 is removed and this stream is remixed with the unshifted stream and recycle gas in the methanol loop to give a balanced gas for methanol synthesis. Purge gas from the recycle loop and the rejected CO.sub.2 from the CO.sub.2 removal section are sent to the gas turbine. The use of a conventional, gas-phase methanol synthesis reactor in an IGCC coproduct scheme is subject to the same shortcomings as in a gas-phase all-methanol product plant: a shift section and CO.sub.2 removal section are required in order to achieve a feed gas composition with an "R" value greater than 2.0, shift and methanol synthesis are performed in separate vessels, and the conversion per pass is limited by temperature constraints.
The liquid-phase methanol process has an advantage over gas-phase methanol synthesis in a coproduct configuration because of its ability to directly process CO-rich gas (e.g., "R" values between about 0.30 and 0.40). The entire CO-rich gas stream from the gasifier is sent through the liquid-phase reactor in a single pass, achieving 10-20% conversion of CO to methanol. While additional methanol can be produced by balancing the gas prior to feeding it to the liquid-phase methanol reactor, the value of this incremental methanol is outweighed by the cost of separate shift and CO.sub.2 removal units. Because a liquid-phase methanol reactor operates isothermally, there is no increasing catalyst temperature and the accompanying constraint on methanol conversion which is characteristic of gas-phase methanol synthesis processes. In a typical liquid-phase design, approximately 14% of the CO (feedgas "R"=0.34) is converted to methanol, giving a reactor effluent containing approximately 9% methanol; the per pass conversion in a gas-phase reactor generally results in a reactor effluent containing only 5% methanol even though the feedgas has an "R" greater than 2.0. It should be noted, however, that even with the superior performance of the liquid-phase reactor, the coproduction scheme can still be expensive, and there is incentive to improve this processing route.
A somewhat similar coproduction scheme is also worthy of mention (U.S. Pat. No. 3,986,349 and 4,092,825). This scheme involves converting coal-derived syngas into liquid hydrocarbons via Fischer-Tropsch synthesis, separating the hydrocarbons from the unreacted gas, feeding the gas to a gas turbine to generate electric power, and using at least part of the hydrocarbons as peak-shaving fuel. Although methanol is mentioned as a possible by-product of the hydrocarbon synthesis, it is not one of the desired products.